herbicide resistance

Earlier this month, I went to Garden Grove, California to attend the 71st annual meeting of the Western Society of Weed Science. My trip was funded by an Education and Enrichment Award presented by the Pahove Chapterof the Idaho Native Plant Society. It was a great opportunity for a weeds-obsessed plant geek like myself to hang out with a bunch of weed scientists and learn about their latest research. What follows are a few highlights and takeaways from the meeting.

General Session

Apart from opening remarks and news/business-y stuff, the general session featured two invited speakers: soil ecologist Lydia Jennings and historian David Marley. Lydia’s talk was titled “Land Acknowledgement and Indigenous Knowledge in Science.” She started by sharing a website called Native Land, which features an image of the Earth overlayed with known “borders” of indigenous territories. By entering your address, you can see a list of the tribes that historically used the land you now inhabit. It is important for us to consider the history of the land we currently live and work on. Lydia then compared aspects of western science and indigenous science, pointing out ways they differ as well as ways they can be used in tandem. By collaborating with tribal nations, weed scientists can benefit from traditional ecological knowledge. Such knowledge, which has historically gone largely unrecognized in the scientific community, should receive more attention and acknowledgement.

David Marley was the comic relief. Well-versed in the history of Disneyland, he humorously presented a series of stories involving its creation. Little of what he had to say related to weed science, which he openly admitted along the way; however, one weeds related story stood out. Due to a lack of funds, the early years of Tomorrowland featured few landscape plants. To make up for that, Walt Disney had signs with fake Latin names created for some of the weeds.

Weeds of Range and Natural Areas

I spent the last half of the first day in the “Weeds of Range and Natural Areas” session where I learned about herbicide ballistic technology (i.e. killing plants from a helicopter with a paintball gun loaded with herbicide). This is one of the ways that Miconia calvescens invasions in Hawaii are being addressed. I also learned about research involving plant debris left over after logging. When heavy amounts of debris are left in place, scotch broom (Cytisus scoparius) infestations are thwarted. There was also a talk about controlling escaped garden loosestrife (Lysimachia punctata) populations in the Seattle area, as well as a few talks about efforts to control annual grasses like cheatgrass (Bromus tectorum) in sagebrush steppes. Clearly there are lots of weed issues in natural areas, as that only covers about half the talks.

Basic Biology and Ecology

On the morning of the second day, the “Basic Biology and Ecology” session held a discussion about weeds and climate change. As climate changes, weeds will adapt and find new locations to invade. Perhaps some weeds won’t be as problematic in certain areas, but other species are sure to take their place. Understanding the changes that are afoot and the ways that weeds will respond to them is paramount to successful weed management. This means documenting the traits of every weed species, including variations between and among populations of each species, so that predictions can be made about their behavior. It also means anticipating new weed species and determining ways in which weeds might exploit new conditions.

No doubt there is much to learn in order to adequately manage weeds in a changing climate. An idea brought up during the discussion that I was particularly intrigued by was using citizen scientists to help gather data about weeds. Similar to other organizations that collect phenological data from the public on a variety of species, a website could be set up for citizen scientists to report information about weeds in their area, perhaps something like this project in New Zealand. Of course, there are already a series of apps available in North America for citizen scientists to report invasive species sightings, so it seems this is already happening to some degree.

Teaching and Technology Transfer

A highlight of the afternoon’s “Teaching and Technology Transfer” session was learning about the Wyoming Restoration Challenge hosted by University of Wyoming Extension. This was a three year long contest in which thirteen teams were given a quarter-acre plot dominated by cheatgrass with the challenge to restore the plant community to a more productive and diverse state. Each team developed and carried out their own strategy and in the end were judged on a series of criteria including cheatgrass and other weed control, plant diversity, forage production, education and outreach, and scalability. Preliminary results can be seen here; read more about the challenge here and here.

And so much more…

Because multiple sessions were held simultaneously, I was unable to attend every talk. I also had to leave early on the third day, so I missed those talks as well. However, I did get a chance to sit in on a discussion about an increasingly troubling topic, herbicide-resistant weeds, which included a summary of regional listening sessionsthat have been taking place in order to bring more attention to the subject and establish a dialog with those most affected by it.

Weeds are “the single greatest threat to agricultural productivity worldwide, costing an estimated $33 billion per year in the United States alone.” Understanding the origins, population structures, and genetic compositions of agricultural weeds will not only help us better mitigate current weed problems but may also help prevent the development of future weed species.

In the introduction, the authors present three modes of weed origination: 1. De-domestication (“domesticated species becoming feral”) 2. Hybridization of domesticated species with related wild species 3. Expansion of wild plants into agricultural ecosystems “through plasticity, adaptation, or exaptation [a shift in function of a particular trait].” In this study, the authors focused on the third mode – the wild-to-weed pathway – claiming that it receives “less attention by evolutionary biologists, even though all weeds without close crop relatives must have followed this pathway to agricultural invasion, and even though this type of weed species is the most common.” Due to the dearth of research, there are several questions yet to be fully addressed: Does invasion require evolutionary changes in the plant and/or changes in agricultural practices? What is more common, single or multiple wild sources? What are the morphological, physiological, and ecological traits that might “predispose a wild species to expand into agricultural habitats?”

To help answer these questions, the authors turned to waterhemp (Amaranthus tuberculatus), a weed that, since first invading agricultural land in the 1950’s, has “become a major problem for corn and soybean farmers in Missouri, Iowa, and Illinois.” Waterhemp is native to the midwestern United States, where it can be found growing along riverbanks and in floodplains. It is a small seeded, dioecious (“obligately outcrossing”), wind-pollinated, annual plant with fruits that can be either dehiscent or indehiscent. Herbicide resistance has been detected in A. tuberculatus for at least six classes of herbicides, making it a difficult weed to control.

There is evidence that A. tuberculatus was previously in the process of diverging into two species, an eastern one and a western one, geographically separated by the Mississippi River. However, “human disturbance brought the taxa back into contact, and possibly gave rise to the agriculturally invasive strain through admixture.” Using population genetic data, the authors set out to determine if the present-day species would show evidence of a past divergence in progress prior to the 20th century. They also hypothesized that “the agricultural weed originated through hybridization between the two diverged lineages.”

After genotyping 38 populations from across the species range, the authors confirmed that A. tuberculatus was indeed diverging into two species. Today, the western variety (var. rudis) has expanded eastward into the territory of the eastern variety (var. tuberculatus), extending as far as Indiana. Its expansion appears to be facilitated by becoming an agricultural weed. Data did not confirm the hypothesis that the weedy strain was a hybridized version of the two varieties, but instead mainly consists of the western variety, suggesting that “admixture is not a pre-requisite for weediness in A. tuberculatus.”

Further investigation revealed that the western variety may have already been “genetically and phenotypically suited to agricultural environments,” and thus did not require “genetic changes to be successful” as an agricultural weed. “Finer-scale geographic sampling” and deeper genetic analyses may help determine whatever genetic basis there might be for this unfortunate situation.

This paper looks at an agricultural weed that originated from the de-domestication of a crop plant (one of the three modes of weed origination stated above). A weed that belongs to the same species as the crop it invades is referred to as a conspecific weed, and weedy rice is “one of the most devastating conspecific weeds in the United States.” Oryza sativa is the main species of rice cultivated in the US, and most varieties are from the group tropical japonica. The two main varieties of weedy rice are straw hull (SH) and black-hull awned (BHA), which originated from cultivated varieties in the groups indica and aus respectively. Because weedy rice is so closely related to cultivated rice, it is incredibly difficult to manage, and there is concern that cross-pollination will result in the movement of traits between groups. For this reason, the authors of this study investigated flowering times of each group in order to assess the “extent to which flowering time differed between these groups” and to determine “whether genes affecting flowering time variation in rice could play a role in the evolution of weedy rice in the US.”

Crop plants have typically been selected for “uniformity in flowering time to facilitate harvesting.” The flowering time of weed species helps determine their effectiveness in competing with crop plants. Flowering earlier than crop plants results in weed seeds dispersing before harvest, “thereby escaping into the seed bank.” Flowering simultaneously with crop plants can “decrease conspicuousness, and seed may be unwittingly collected and replanted” along with crop seeds. Simultaneous flowering of weeds and crops is of special concern when the two are closely related since there is potential for gene transfer, especially when the crop varieties are herbicide resistant as can be the case with rice (“60-65% of cultivated rice in [the southern US] is reported to be herbicide resistant”).

For this study, researchers observed phenotypes and gene regions of a broad collection of Oryza, including cultivated varieties, weed species, and ancestors of weed and cultivated species. They found that “SH weeds tend to flower significantly earlier than the local tropical japonica crop, while BHA weeds tend to flower concurrently or later than the crop.” When the weeds were compared with their cultivated progenitors, it was apparent that both weed varieties had “undergone rapid evolution,” with SH weeds flowering earlier and BHA weeds flowering later than their respective relatives. These findings were consistent with analyses of gene regions which found functional Hd1 alleles in SH weeds (resulting in day length sensitivity and early flowering under short-day conditions) and non-functional Hd1 alleles in BHA weeds (“consistent with loss of day-length sensitivity and later flowering under short-day conditions”). However, the authors determined that there is more to investigate concerning the genetic basis of the evolution of flowering time in weedy rice.

In light of these results, hybridization is of little concern between cultivated rice and SH weeds. BHA weeds, on the other hand, “have a greater probability of hybridization with the crop based on flowering time and Hd1 haplotype.” The authors “predict that hybrids between weedy and cultivated rice are likely to be increasingly seen in US rice fields,” which, considering the current level of herbicide resistant rice in cultivation, is quite disconcerting.